Vertical reference devices find use in numerous commercial and military applications. For example, they can be used to provide sensory input to the microprocessors and motors used for keeping a platform on a ship, military, commercial, or otherwise, at a predetermined orientation relative to some predetermined frame and/or axis of reference, often the gravitational vector and a plane parallel thereto. Thus, they have been used to keep decks on luxury yachts at a constant, level or non-tilting orientation relative to the horizon; to keep decks on cattle ships at the same orientation, thereby avoiding making the animals sick during ocean crossings; and to control the orientation of helipads on military ships so that the pad is not tilting with the ship as helicopters land on the pad.
Vertical reference devices utilized in such applications incorporate a variety of sensing devices whose output signals are processed to provide a signal to a microprocessor that drives one or more motors or hydraulic cylinders that are used to control the orientation of the platform. One form of such a device is shown in U.S. Pat. No. 3,974,699 to Morris. Morris utilizes linear and angular accelerometers to provide input signals to an electronic system that provides an output signal to a control system used to keep particular structures, such as a motor grader blade or a train car passenger cabin, in a predetermined orientation. Another Morris patent, U.S. Pat. No. 4,038,876, also uses linear and angular accelerometers to provide inputs signals to a control system utilized to monitor and stabilize a structural body about at least one axis. Morris notes the applicability of this device to ocean based platforms such as off-shore oil drilling platforms and to stable platforms and "strapdown" situations.
Using accelerometers as Morris does, however, creates problems with a drift of the output signal. This occurs because of the dual integration that must be performed on the accelerometer signals provided by those devices before the output signal derived therefrom is provided to the appropriate control systems that are used to drive the motors and/or hydraulic devices used to stabilize the platform or other structure at the predetermined orientation. The Morris devices lack accuracy in an environment where lateral accelerations are commonplace.
Other sensors, such as angular rate sensors and liquid pendulums can be used in place of accelerometers to provide the sensory inputs used by a vertical reference to provide an output signal to a control system. In constructing such an instrument, inertial vertical accuracy response is desirable but generally unobtainable with presently known devices because of pendulum errors introduced by lateral accelerations. To improve the response curve overall, it would be better if the pendulum response had a steeper cut off slope than that provided by presently known devices, thus reducing the errors caused by lateral accelerations experienced by the vertical reference.
It would be desirable to have a vertical reference that provided an output signal that was less subject to drift due to multiple integrations of sensor signals and that produced a more accurate output or control signal under environments subject to lateral accelerations.